Subject: Why does calcium chloride become warm when added to water?

We did a science activity in class involving a solution of calcium 
chloride and a solution of magnesium sulfate.  When the calcium chloride 
was added to the water and mixed around the test tube got warm. The 
Magnesium sulfate didn't.  I thought heat was only produced by a chemical 
reaction.  What chemical reaction is going on when calcium chloride is 
added to water? Does something different happen to the calcium chloride 
than to the magnesium sulfate?

Re: Why does calcium chloride become warm when added to water?